1. Field of the Invention
The present invention relates to a method for packet transmission and prioritization, and more particularly, to a method of performing transmission and prioritization for packets of a radio link control (RLC) layer for a medium access control (MAC) layer of a wireless communications system.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs).
A radio link control (RLC) layer is responsible for data transfer of radio bearers from an upper layer, Packet Data Convergence Protocol (PDCP) layer, and includes three transfer modes of Transparent Mode (TM), Unacknowledged Mode (UM) and Acknowledged Mode (AM). An RLC entity exchanges RLC service data units (SDUs) with upper layer and RLC protocol data units (PDUs) with its peer RLC entity via lower layers. An RLC PDU can either be an RLC data PDU or an RLC control PDU.
In the LTE system, a UM RLC entity is configured either as a transmitting UM RLC entity or a receiving UM RLC entity. The transmitting UM RLC entity receives RLC SDUs from upper layer and sends RLC PDUs to its peer receiving UM RLC entity via lower layers. The receiving UM RLC entity delivers RLC SDUs to upper layer and receives RLC PDUs from its peer transmitting UM RLC entity via lower layers. In addition, the transmitting UM RLC entity segments and/or concatenates RLC SDUs in accordance to a transport block (TB) size selected by lower layer at the particular transmission opportunity when forming UM data (UMD) PDUs from the RLC SDUs.
In the LTE system, an AM RLC entity, either in the E-UTRAN or in the UE, consists of a transmitting side and a receiving side and supports segmentation, retransmission, sequence check and other functions. The receiving side of the AM RLC entity delivers RLC SDUs to the upper layers and receives RLC PDUs from its peer AM RLC entity via the lower layer. The transmitting side of the AM RLC entity receives RLC SDUs from upper layers and delivers RLC PDUs to its peer AM RLC entity according to a TB size indicated by a medium access control (MAC) layer. In addition, the transmitting side of the AM RLC entity segments and/or concatenates RLC SDUs in accordance to the TB size selected by the MAC layer at the particular transmission opportunity when forming AM data (AMD) PDUs from the RLC SDUs.
In AM data transfer, the transmitting side of the AM RLC entity shall prioritize transmission of RLC control PDUs over RLC data PDUs and further prioritize retransmission of RLC data PDUs over transmission of new AMD PDUs. In retransmission, the transmitting side of an AM RLC entity delivers the AMD PDU if the AMD PDU can entirely fit into the TB of the particular transmission opportunity, otherwise segments the AMD PDU to form a new AMD PDU segment which can fit into the TB.
The MAC layer, a lower layer of the RLC layer supports functions of mapping between logical channels and transport channels, multiplexing, de-multiplexing, logical channel prioritization, transport format selection, and so on. An MAC entity of the MAC layer, performing the MAC functions, exchanges RLC PDUs, seen as MAC SDUs, with the RLC layer via logic channels and MAC PDUs with the physical layer via transport channels, such as an uplink shared channel (UL-SCH) or a downlink shared channel (DL-SCH). An MAC SDU can be an RLC data PDU, an RLC data PDU segment or an RLC control PDU. More specifically, the MAC entity multiplexes MAC SDUs from one or different logical channels onto the TB to be delivered to the physical layer on transport channels. Each MAC transmission is allocated with a transmission time interval (TTI).
In the logical channel prioritization of the MAC layer, a logical channel prioritization procedure is applied when a new transmission of a hybrid automatic repeat request (HARQ) process is performed. Each logical channel is given a Prioritized Bit Rate (PBR). The PBR is a rate like X bytes/s or Y bits/s. The logical channel prioritization procedure ensures that all the logical channels are served in a decreasing priority order up to their configured PBR, and if any resources remain, all the logical channels are served in a strict decreasing priority order until either the data for that logical channel or the UL grant is exhausted, whichever comes first.
A MAC PDU consists of a MAC header, zero or more MAC SDUs, zero or more MAC control elements, and optionally padding. Both the MAC header and the MAC SDUs are of variable sizes. The MAC PDU header consists of one or more MAC PDU sub-headers, each corresponding to either a MAC SDU, a MAC Control element or padding. MAC PDU sub-headers have the same order as the corresponding MAC SDUs, MAC Control elements and padding. The MAC control elements are always placed before any MAC SDU and padding. A maximum of one MAC PDU can be transmitted per TB per UE, and one or two TBs can be transmitted per TTI per UE.
According to the prior art, the MAC selects a TB size for an AM RLC entity, and the TB, onto which the MAC SDUs are multiplexed from one or different logical channels, has the same size as the MAC PDU does. In this situation, a problem of TB allocation for RLC entities is incurred in a scenario where more than one RLC entity have data to transmit and logical channels of these RLC entities are multiplexed in the DL-SCH or UL-SCH. For example, two RLC entities of a UE with equal logical channel priority have data to transmit and two logical channels of these RLC entities are multiplexed in UL-SCH. The MAC layer has to provide half the TB size for each of the RLC entities when both of the RLC entities have data, more than half the TB size, to transmission. Therefore, it is unreliable for the RLC SDUs to be segmented and/or concatenated only in accordance to the TB size selected by MAC. Also, it is unreliable for the AMD PDU for retransmission to be determined whether to be segmented only depended on whether the AMD PDU can entirely fit into the corresponding TB or not.
The prior art does not clearly specify how to decide the RLC data PDU size when the RLC control PDU transmission is transmitted in the same transmission opportunity. Furthermore, the current logical channel prioritization is not clearly defined in the prior art.